This invention relates to a packet switching digital signal communications system, and more particularly to a channel access system for allowing a station attempt ing transmission to establish the right of carrying out transmission on a communications cable such as a coaxial cable.
With the proliferation of electronic computers and the development of digital signal processing techniques, data communications for on-line data processing by combining of communications systems and data processing systems have become very important. Particularly for small scale communications systems such as private communications provided in government and municipal offices and private companies, packet switching communications systems using communications cables such as coaxial cables have attracted much public attention because of their high reliability and economic and transmission efficiency.
In a packet switching communications system, a communications cable for carrying out two-way transmission is laid in a research laboratory or the like, and a number of stations (personal stations) are connected to the cable. Then, a message divided into data blocks of, e.g., 1,000-2,000 bits is transmitted from each station. Addresses, serial numbers and other headers are added to the messages. According to such a communications system, the network itself is a passive transmission medium with no control function, and the control functions have been completely decentralized in the hands of the stations. Accordingly, each station gains access to a channel after confirming that the transmission line is idle and starts transmitting a message. Should a collision of packets occur during transmission, both stations stop their transmissions. The stations which have stopped their transmissions are allowed to attempt to transmit messages again after a random access time.
Since each station starts transmitting its data according to this communications system, packet collisions may frequently occur on any given transmission line. Consequently, there is a problem that the transmission delay time will not be constant. Also, the transmission system becomes unacceptable for real time transmission in which emphasis is placed on the time relationship between transmission and reception, such as in aural communications of an interface type. Naturally, this problem may be solved by providing a master station and by making each station secure the channel access in advance. However, such an arrangement may make it impossible to carry out data communications when the master station becomes inoperable, thus reducing system reliability.
In order to deal with these disadvantages, a digital signal transmission system called a Modified Ethernet system has been proposed. According to this system, by dividing a large frame periodically repeated on a time axis into a plurality of small frames, or blocks, each personal station is given a chance to carry out packet communication in any of the available blocks. In so doing, each station can use a vacant block on an equal footing and, when a station wishes to monopolize a particular block for a required period of time for signal transmission, a chance to transmit a signal each time the frame is repeated is periodically given to the station. In other words, real time transmission becomes possible.
FIG. 1 shows an arrangement of the signal frame according to the modified ethernet system. The frame periodically repeated on the time axis comprises N blocks #1-#N. Each block consists of various bit strings b1-b9 as follows:
b1: rear guard time; PA2 b2: preamble; PA2 b3: address bits; PA2 b4: distance code bits; PA2 b5: control bits; PA2 b6: data bits; PA2 b7: check bits; PA2 b8: end flag; and PA2 b9: front guard time.
In this case, the bits b2-b5 and b7-b8 are required for a packet configuration and are generally called overhead (additional) bits. In addition, the two bit strings b1 and b9 each provide a guard time. The guard time is an idle bit to eliminate the partial overlapping of adjacent packets caused by the delay time when the packet in each block propagates along a coaxial cable. There are two kinds of guard bits, the rear guard time bits b1 for protecting a packet positioned in the rear of a bit string and the front guard time bits b9 for protecting a packet positioned in the front. The sum of the number of bits of the rear and front guard times b1 and b9 is assumed to be g bits, and the guard time (bl +b9) is represented by tg.
FIG. 2 illustrates in block diagram form the essential components of a modified ethernet communications system. In this system, both ends of a coaxial cable 1 laid as a transmission line are connected to impedance matching terminations 2 having the same resistance as the characteristic impedance of the cable 1. Each station is connected to the coaxial cable 1 through a T connector (tap) 3.sub.1 -3.sub.N. These stations are basically of the same construction, and there is therefore shown the detailed configuration of only the station A connected to the T connector 3.sub.1.
Each station is equipped with user equipment 4 provided with a computer and a telephone. The user equipment 4 has a transmitter (encoder) 41 for transmitting digital signals in packets to the other stations, a receiver (decoder) 42 for receiving signals in packets sent by the other stations, a terminal control unit 43 for controlling a terminal unit, and so forth. The signals outputted from the transmitter 41 are temporarily stored in a transmission buffer memory 51 and are collectively read out at a preset time with a clock signal substantially equal to a transmission speed on the coaxial cable transmission medium. The signals read out are converted into predetermined packets by a transmission logic circuit 52. The signals are then sent to the coaxial cable 1 through the T connector 3.sub.1 after they have passed through a transmission buffer amplifier 53.
On the other hand, all packet signals being transmitted on the coaxial cable 1 are received by a reception buffer amplifier 54 through the T connector 3.sub.1. A reception logic circuit 55 selects only packets addressed to its own station from those received, and temporarily stores the selected packets in a reception buffer memory 56. The stored signals are continuously read out using the predetermined clock, to thereby obtain the reception output.
Signals are thus transmitted and received as described above, and the transmission clock used therein is generated by a transmission clock oscillator 57. A frame counter 58 divides the transmission clock to prepare frame timing and block timing signals 71 and 72 for specifying the frame timing. A transmission control circuit 59 controls the terminal control unit 43 using the received signals destined for its own station obtained from the reception logic circuit 55, and at the same time controls the transmission logic circuit 52 according to the instructions given by the terminal control unit 43. In addition, a collision detection circuit 61 examines whether a first packet signal transmitted in the block chosen by its own station has collided with a first packet of another station.
The collision of packets in the communications system will now be described in further detail. The user equipment 4 is provided with a block status memory (not shown) indicating which of the blocks #1-#N in the frame are occupied. Since the packet signals of each station are received by the reception buffer amplifier 54, the blocks being used can be registered on the basis of these received packet signals. In order to make possible real time transmission in the modified ethernet system, any station which has occupied a certain block is allowed to continuously occupy that block in the next frame. Accordingly, a station which desires transmission selects a vacent block indicated in the memory and sends a packet signal to that block in the next frame. However, if more than one station desires to start transmission at the same time, these stations will select the same vacant block, causing packet signals to be simultaneously generated. A collision will be produced at this time.
FIG. 3 illustrates such a state of collision. Assume that stations are arranged on the coaxial cable with the same relationship between them as that shown in FIG. 2. In other words, the stations A and C are arranged close to either end of the coaxial cable 1, whereas the station B is arranged between them. When the stations A and B attempt to start transmission simultaneously, these stations each expect to start transmitting packet signals to the Mth block #M (1.ltoreq.M.ltoreq.N). In this case, the station A starts transmitting the packet signal P-A with the predetermined block timing. The packet signal P-A is received by the station B slightly later due to the propagation delay time on the coaxial cable, and still later by the station C. On the other hand, the packet signal P-B transmitted by the station B is sent out later by a predetermined period of time than the time at which the packet signal P-A is sent out. The reason for this is that packet signals are prevented from partially overlapping one another between the blocks by arranging the packet signals of adjacent blocks at a certain interval equivalent to the sum .tau.g of the guard time at the middle point of the coaxial cable. The packet signal P-B is also received by the stations A and C after some respective 10 delays.
A collision detection circuit 61 at the station A detects the collision at the point of time that the packet signal P-B is received thereby. In concert with this detection, the transmission of the packet signal P-A is stopped halfway because, if more than one packet signal is mixed with another, it will contain no meaningful data. In the same way, the station B stops transmitting the packet signal P-B upon receiving the packet signal P-A. These stations each select a vacant block after some random access time, and each is allowed to transmit its packet signal again. At this time, the existing block #M is returned to an unused state thereafter, because each of the stations ceases to transmit a signal. The higher the frequency of transmission demand made in one communications system, the higher the probability that a plurality of stations will try to gain access to one and the same vacant block. In this case, the number of packet signal collisons increases, causing a delay in the establishment of a call, and the number of stations trying to retransmit their packet signals again increases and this results in an increase in the number of blocks at which collisions occur. In other words, the channel utilization ratio will be reduced, and a further disadvantage is that a period between the time the transmission demand is made and the time (transmission delay time) at which the transmission of a packet has been successful is increased.